1. Field of the Invention
The present invention relates to downhole tools used in subsurface well completion pumping operations, and particularly to tools used to enhance the effectiveness of gravel pack operations.
2. The Related Art
Gravel packing is a method commonly used to complete a well in which the producing formations are loosely or poorly consolidated. In such formations, small particulates referred to as “fines” (e.g., formation sand) may be produced along with the desired formation fluids. This leads to several problems such as clogging the production flowpath, erosion of the wellbore, and damage to expensive completion equipment. Production of fines can be reduced substantially using a steel wellbore screen in conjunction with particles sized to prevent passage of fines such as sand through the screen. Such particles, referred to as “gravel,” are pumped as gravel slurry into an annular region between the wellbore and the screen. The gravel, if properly packed, forms a barrier to prevent the fines from entering the screen, but allows the formation fluid to pass freely therethrough and be produced.
A common problem with gravel packing is the presence of voids in the gravel pack. Voids are often created when the carrier fluid used to convey the gravel is lost or “leaks off” too quickly. The carrier fluid may be lost either by passing into the formation or by passing through the screen where it is collected by the end portion of a service tool used in gravel pack applications, commonly known as a wash pipe, and returned to surface. It is expected and necessary for dehydration to occur at some desired rate to allow the gravel to be deposited in the desired location. However, when the gravel slurry dehydrates too quickly, the gravel can settle out and form a “bridge” whereby it blocks the flow of slurry beyond that point, even though there may be void areas beneath or beyond it. This can defeat the purpose of the gravel pack since the absence of gravel in the voids allows fines to be produced through those voids.
Another problem common to gravel packing horizontal wells is the sudden rise in pressure within the wellbore when the initial wave of gravel, the “alpha wave,” reaches the far end or “toe” of the wellbore. The return or “beta wave” carries gravel back up the wellbore, filling the upper portion left unfilled by the alpha wave. As the beta wave progresses up the wellbore, the pressure in the wellbore increases because of frictional resistance to the flow of the carrier fluid. The carrier fluid not lost to the formation conventionally must flow to the toe region because the wash pipe terminates in that region. When the slurry reaches the upper end of the beta wave, the carrier fluid must travel the distance to the toe region in the small annular space between the screen and the wash pipe. As this distance increases, the friction pressure increases, causing the wellbore pressure to increase.
The increased pressure can cause early termination of the gravel pack operation because of the risk that the wellbore pressure can rise above the formation fracture pressure, causing damage to the formation and leading to a bridge at the fracture. Thus, gravel pack operations are typically terminated when the wellbore pressure approaches the formation fracture pressure. Such early termination can lead to an incomplete packing of the wellbore and leave undesirable voids in the gravel pack. The only viable alternative is to redesign the gravel pack, and bear the attendant increases in time and expense, when the annulus pressure is expected to approach the formation fracture pressure.
Thus, a need exists to control the pressure in the wellbore resulting from progression of the carrier fluid beta wave. More particularly, a need exists for maintaining the annulus pressure below the formation's intrinsic fracture pressure on a real-time basis. Additionally, such pressure control should be compatible with subsequent fluid pumping/flow operations that typically following gravel packing.